Internal combustion engines of vehicles may have operating modes which may be selectable by the driver. Thus in some vehicles an economy mode, a normal mode and a sport mode may be provided, each mode having a different engine response to one or more commands of the driver. Typically the engine may respond differently in each mode to a given input of the accelerator pedal, being least responsive in economy mode, and most responsive in sport mode. In this way driveability of the vehicle can be improved by providing a range of accelerator pedal movement which is appropriate to, for example, the desired output torque characteristic of the engine. Such a system necessarily relies upon an electronic input from the vehicle driver, for example an accelerator potentiometer providing an input signal to an electronic control unit having a plurality of accelerator pedal position/output torque maps. The maps may also be referred to as accelerator pedal progression maps or pedal progression maps.
Another kind of operating mode relates to the terrain which a vehicle is intended to cross. U.S. Pat. No. 7,349,776, the content of which is hereby incorporated by reference, describes a vehicle control system in which the driver can implement improved control over a broad range of driving conditions, and in particular over a number of different terrains which may be encountered when driving off-road. In response to a driver input command relating to the terrain, the vehicle control system is selected to operate in one of a number of different driving modes including one or more terrain response (TR) modes. For each TR mode, the various vehicle subsystems are operated in a manner appropriate to the corresponding terrain.
In one arrangement, a mode (for example a winter mode) is available in which the vehicle is configured to launch from standstill in a forward gear other than first gear such as a second gear to reduce risk of excessive wheel slip. Different modes may have different accelerator pedal maps (amount of engine torque developed for a given accelerator pedal position), torque delivery (accelerator pedal maps in combination with a rate at which engagement of transmission clutches is controlled to take place thereby determining how aggressively gear shifts take place), and transmission shift points as a function of coefficient of surface friction. For example, in one or more TR modes the transmission shift points may be arranged wherein gear shifts take place at lower speeds than they might otherwise take place at, and in a more gentle manner (e.g. at a slower rate).
Thus one or more accelerator/torque maps appropriate to the terrain may be selectable by the driver. For example when driving in rocky terrain, a high torque output may be indicated for a small accelerator movement, thus giving immediate urge to overcome a rock step. In contrast on sand, a low torque output may be indicated for the same accelerator movement, so as to avoid spinning a vehicle wheel and digging a hole. To some extent the selected torque map may be a matter of judgement related to the available grip on the terrain.
All torque maps (pedal progression characteristics) associated with any vehicle operating mode coincide at minimum accelerator position/zero torque and maximum accelerator position/maximum torque. In between these conditions, a change of map causes an immediate change of engine output torque which may be disconcerting to the vehicle driver, especially if the accelerator pedal is not being moved at the time of change.
A change of torque map may also be undesirable at minimum accelerator position if the behaviour of the vehicle is not as expected when the accelerator pedal is next advanced.
If an alternative operating mode is selected by the vehicle driver, a change in the characteristic of engine response is generally not a surprise—the change is in fact expected by the driver and is generally desirable. However difficulties may arise if the operating mode is automatically selected in response to a vehicle sensing a change of operating condition. Thus, for example a vehicle may detect a change of terrain from rock to sand and, whilst the accelerator is partly applied, command the vehicle engine to adopt a different torque map. A consequent instant change in engine response may be disconcerting to the driver, especially if such automatic mode changes are repeated frequently.
Likewise a change of mode as the vehicle is rolling to a halt (minimum accelerator pedal position) may mean that the next depression of the accelerator causes the vehicle to accelerate significantly differently to expectation.
FIG. 1 shows a known motor vehicle 101 having a powertrain 101P. The powertrain 101P includes an engine 121, a transmission 124, a power take-off unit (PTU) 137, a rear driveshaft or propshaft 131R and a front driveshaft or propshaft 131F. The rear driveshaft 131R is operable to drive a pair of rear wheels 113, 114 via a rear differential 135R whilst the front driveshaft 131F is operable to drive a pair of front wheels 111, 112 via a front differential 135F.
The vehicle 101 has an engine controller 121C arranged to receive an accelerator pedal position signal from an accelerator pedal 161 and a brakes controller 141C operable to receive a brake pedal position signal from a brake pedal 163.
In the configuration of FIG. 1 the transmission 124 is releasably connectable to the rear driveshaft 131R by means of the power transfer unit (PTU) 137, allowing selectable two wheel drive or four wheel drive operation.
The PTU 137 is also operable in a ‘high ratio’ or a ‘low ratio’ configuration, in which a gear ratio between an input shaft and an output shaft thereof is selected to be a high or low ratio. The high ratio configuration is suitable for general on-road or ‘on-highway’ operations whilst the low ratio configuration is more suitable for negotiating certain off-road terrain conditions and other low speed applications such as towing.
The vehicle 101 has a central controller 101C, referred to as a vehicle control unit (VCU) 101C. The VCU 101C receives and outputs a plurality of signals to and from various sensors and subsystems provided on the vehicle 101.
The vehicle 101 has a transmission mode selector dial 124S operable to select a required operating mode of the transmission 124. The selector dial 124S provides a control signal to a transmission controller 124C which in turn controls the transmission 124 to operate according to the selected mode. Available modes include a park mode, a reverse mode and a drive mode.
The vehicle 101 also has a terrain response mode selector dial 128S. The terrain response mode selector dial 128S is operable by a driver to select a required terrain response mode of operation of the vehicle.
It is to be understood that if a user selects the drive mode of the transmission 124, the engine controller 121C employs a drive mode throttle map to determine the amount of drive torque that the engine 121 should produce as a function of accelerator pedal position. If the user selects a ‘dynamic’ TR mode, the engine controller 121C employs a sport mode accelerator (or throttle) pedal progression map instead of the drive mode accelerator pedal progression map. The throttle maps differ in that the sport mode throttle map is arranged to provide a more aggressive response by the engine 121C to a given initial advance (such as depression) of the accelerator pedal 161.
Different throttle maps are also employed for different respective user-selectable terrain response modes.
As noted above, in some arrangements the vehicle may be operable automatically to select an appropriate TR mode for the prevailing driving conditions.
FIG. 2 shows two different accelerator pedal progression maps in the form of a plot of engine torque output T as a function of accelerator pedal position P on a scale from 0 to 100% of full scale depression of the accelerator pedal 161. Two extreme vehicle operating modes A,B are shown. Mode A is an initially cautious torque map and may correspond for example to a TR mode suitable for use when driving over sand. Mode B is a more aggressive torque map and may correspond to a TR mode suitable for use when driving over rock. A driver may select operation according to mode A or mode B my means of the TR mode selector dial 128S. At the zero and 100% accelerator positions, the torque maps coincide, but at part depression of the accelerator pedal significant differences in torque output are apparent.
Thus a switch from mode A to mode B at point C (50% application of accelerator pedal) results in an immediate jump to point D, with consequent increase in engine torque output. The characteristic of line B is subsequently followed whilst mode B is selected. A corresponding switch in the reverse direction to mode A results in significant drop in output torque. Changes between torque maps generally comprise movements in the direction of the y axis.
The change in output torque of the engine may take time, and can be deliberately blended, as illustrated in FIG. 3. Thus the increase from point C to point D may be controlled to avoid a step change. For example a maximum blending rate, say 7 Nm/s may be applied, and/or blending may take place at a defined rate within a maximum time period, of say 20 seconds. A small torque change will blend quickly, and a large torque change will take longer.
FIG. 4 illustrates a blending at a calibrated rate of, say, 7 Nm/s from a cautious torque map A to an aggressive torque map B. At each time interval t=1, t=2 etc., the cautious map approaches the aggressive map, but the shape of the cautious torque map is maintained until blending is complete, whereupon the characteristic follows the aggressive map B. Blending is generally in the direction of arrow E. Thus a change in accelerator position whilst blending is in progress does not change the sensitivity thereof.
Although blending is progressive, nevertheless the vehicle driver will still be subject to a sudden difference in sensitivity of the accelerator pedal, as the characteristic switches from the shape of the cautious map to the aggressive map, when blending is complete. Thus a blend may be in progress and have reached point F. Application of the accelerator pedal will result in an engine torque following the shape of the cautious map to point G, and then to follow the aggressive map towards point H. The step change in pedal response at point G may be disconcerting to the driver, since further application of the accelerator pedal suddenly has little effect upon engine torque output in this example.
What is required is a blending strategy that allows for switching between different torque maps without the vehicle driver being presented with a significant and unexpected change in accelerator pedal response. Such a strategy is suitable for a vehicle having manual switching between operating modes, and thus torque maps, and also where switching is automatic.
It is against this background that the present invention has been conceived. Aspects and embodiments of the invention may provide a method, a system or a vehicle in which blending of torque is improved. Other aims and advantages of the invention will become apparent from the following description, claims and drawings.